The present invention relates to a fluxgate micro-power magnetometer and more particularly to an oscillator driver portion thereof for decreasing the power required for core saturation therein to a range of 50 to 150 microwatts.
Fluxgate magnetometers in general require that electrical current be supplied periodically to a winding to magnetically saturate one or more magnetic cores. The presence of an applied magnetic field such as the earth's field or the field of a magnetic body such as a motorized vehicle is detected by an additional signal produced on sensor windings wound on the core as the magnetic material of the core cycles in and out of saturation and exhibits non-linear magnetic permeability.
A certain amount of energy E must be supplied in each cycle to magnetically saturate the magnetic material. Material volume and other material parameters as well as geometry and drive circuit efficiency control the magnitude of this energy. Expenditure of energy E in a fluxgate magnetometer permits one sample of the ambient magnetic field to be taken and a numerical value assigned to it through a calibration procedure. This numerical value varies from cycle to cycle because of fluctuations in the magnetic material parameters and in the circuitry supplying the saturating drive current. If n samples per second are taken and averaged, a more stable value for the field measurement is obtained in some desired bandwidth. For a specified bandwidth and field measurement stability (noise) some power nE must be continuously expended to operate the fluxgate magnetometer. For the value of nE to be a minimum it is important to select an appropriate magnetic core and to optimize drive circuit efficiency and stability. The magnetometer of the present invention optimizes drive circuit efficiency and stability and also provides for selection of n and E independently.
Prior art magnetometers include devices such as the two core gradiometer disclosed in Brown, U.S. Pat. No. 3,649,908, wherein a low-noise flyback oscillator/driver, similar to FIG. 1 of the present application supplies unidirectional current to the drive windings to drive the cores around a portion of the hystersis curve. The oscillator/driver circuit is controlled by feedback windings around both cores. A pair of sensor windings are wound on each core and each sensor winding carries a signal composed of the induced drive signal and a magnetic field induced signal that varies with the ambient field. Although the oscillator/driver circuit of Brown has many advantages, the repetition rate, n, of core saturation is controlled, for a given V, largely by the inductance of the drive windings and to some extent by stray capacitances in the circuit. Seeking to reduce magnetometer power consumption significantly through lowering n in the nE product leads to an unacceptably large number of turns on the drive winding if its inductance is increased sufficiently. Not only does an increased number of turns on the drive winding add to cost, the oscillator/driver efficiency and stability are reduced because of increased winding resistance, increased winding capacitance and decreased turns coupling to the core.